1. Field of the Invention
The present invention relates in general to ellipsometers and in particular to a novel ellipsometer using an imaging spectrograph that encodes polarization in one axis and wavelength in the other axis to enable complete simultaneous measurements of the ellipsometric parameters for the full wavelength range of interest. The ellipsometer can be used both in-situ and in an in-line process to give real-time values.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Ellipsometry is the systematic study of the change in the state of polarization of light upon reflection and/or transmission by any substance. Ellipsometers are optical instruments that measure the change of polarization caused by some characteristic of a sample from which the light is being reflected. To fully characterize the change in polarization, the initial state of polarization needs to be known as well as the wavelength. Knowledge of the change of state of polarization upon reflection or transmission significantly constrains the structure of a sample being studied. For example, in semiconductor fabrication, thin films are deposited on the wafer. The thickness of the films may be readily determined from ellipsometric measurements. There are numerous manufacturing processes that require deposition of thin films. Ellipsometers are routinely used to measure the quality of the process by analyzing the film properties.
Ellipsometers that use a single wavelength of light, such as a laser source, and measure certain important polarization angles are often referred to as "nulling ellipsometers". A measurement from a nulling ellipsometer results in a determination of the ellipsometric parameters known as .psi. (psi) and .DELTA. (delta). The ellipsometric parameters are functions of wavelength, angle of incidence, and surface properties of the sample. The parameter .psi. is a measure of the relative intensities of the p-to-s polarization states of the probe light beam and the parameter of .DELTA. is a measure of the relative phase shift between the p and s polarization states. Additional information can be determined about the sample under study by making .psi. and .DELTA. measurements for several different incident angles and/or several different wavelengths. Ellipsometers that operate with a fixed wavelength but vary the angle are called "variable angle ellipsometers" (VAE). Ellipsometers that vary the wavelength (or use a white light probe beam source for the spectrograph) are called "spectral ellipsometers" (SE).
Surface properties generally are not uniform but vary from place-to-place over the sample. There are a variety of techniques to map the properties of the surface as measured by ellipsometry. One way is to have the ellipsometer probe beam constrained to a small spot and then translate the sample in the horizontal plane taking numerous measurements that can be formed into a map of the surface properties. There are also techniques that use a broad beam with multiple detectors, each of which has a one-to-one correspondence with a small spot on the surface.
Prior art ellipsometers implement serial measurements of the ellipsometric parameters as a function of wavelength for a fixed incident angle. Further, because of their speed of operation and cost, they are not generally used "in-situ" or in an "in-line" process.
Use of ellipsometry to monitor a process that builds or removes some material on a surface places certain constraints on the operation of the ellipsometer. In addition to being adequately sensitive to the materials in question, the measurements must be made in a timely manner so that the information is available as needed. Current art in ellipsometric measurement is not particularly suited for high-speed throughput that is greater than about one or two measurements fits of the full spectral ellipsometric parameters per second. It is usually the case that a full measurement of the spectral ellipsometric parameters takes several seconds. Constraints on the speed of operation can usually be traced to the requirement of having to move some mechanical component through numerous discrete positions in order to make a range of measurements and/or that the measurements are made sequentially for each wavelength of light in the probe beam.
It would be advantageous to have an ellipsometer that makes multiple simultaneous measurements of the spectral ellipsometric parameters at all wavelengths simultaneously without the need of any time-varying, or mechanically-moving optical elements.
It would also be advantageous to have an ellipsometer that can take measurements sufficiently fast that it could operate in an in-situ process to determine the characteristics of a sample material as it is being formed or in-line to implement process control.